1. Field of the Disclosure
The present disclosure relates to a rotary friction welding process.
2. Description of the Related Art
Rotary friction welding is the process for welding together two bodies or workpieces by converting mechanical energy to heat energy by the friction between the engaging weld surfaces of the two workpieces. The process involves effecting relative rotation between the two workpieces while the weld surfaces remain in engagement with each other.
For example, in inertia friction welding one of two coaxial workpieces is attached to a flywheel, rotated to a predetermined speed and then driven against the second workpiece using thrust supplied by the welding machine. A fixed amount of stored energy in the flywheel (proportional to rpm2.I, where rpm is the flywheel's predetermined speed and I is its rotational inertia) is thereby converted to heat by friction at the interface of the engaging weld surfaces, which bonds the workpieces together.
The initial contact between the weld surfaces produces a conditioning period in which friction raises the temperature at the interface. This is followed by upsetting when the temperature reaches a high enough level such that softening/melting of the workpiece material allows the workpieces to be pushed together, with liquid or quasi-liquid material being expelled sideways from a plasticised zone at the interface.
In its application to turbine hardware, such as the joining of compressor discs, the weld geometry is tubular. When using rotary friction welding to join two tubes together, it is standard practice for the starting weld surfaces to be flat and parallel end faces of the tubes.
However, variations in contact conditions at the weld surfaces lead to variability in the welding process upset. For example, due to machining tolerances, residual stress distortions etc., the weld surfaces are generally not completely flat, which leads to non-axisymmetric contact, producing local hotspots at the weld interface. At large diameters in thin walled components such as turbine compressor discs, such non-uniform contact can be exaggerated. This results in variability in the efficiency of local heating during the conditioning period and hence variation in the conditioning duration. In the fixed-energy inertia welding process this leads to variation in total upset and hence fitness for purpose either through reduced integrity at low upset (interface contaminants not fully expelled) or component fit at low or high upset.
Variations in contact conditions at the weld surfaces can also reduce control of defect expulsion. For example, the material may be expelled non-axisymmetrically from the interfacial plasticised zone, with a result that interface contaminants may not be fully removed from all parts of the weld. Accordingly, assumptions about flow and contaminant expulsion may be incorrect, leading to sub-optimal process and component design, or a low integrity product with a reduced life.
As well as non-flat weld surfaces, contact condition variability may also be produced by workpiece diameter mismatch, workpiece eccentricity and lack of workpiece coaxiality. In addition, where a welding machine has a limited thrust capability this can compromise the preferred contact pressure for a given tubular wall thickness.